Longitudinal
Wave:
A wave composed of alternate surfaced of compression and rarefaction
traveling perpendicular to these surfaces. Particle motion is
in the direction of travel.

Transverse
Wave:
The particle displacement at each point in a material is perpendicular
to the direction of wave propagation. Transverse waves are not
supported by liquids and gasses.

Transducer:
A device that converts one form of energy into another. In ultrasonics,
electrical energy is converted to mechanical (sound) energy and
visa versa.

Precision
Velocity Measurements

Changes in ultrasonic wave propagation speed, along with energy
losses, from interactions with a materials microstructures are
often used to nondestructively gain information about a material's
properties. Measurements of sound velocity and ultrasonic wave
attenuation can be related to the elastic properties that can
be used to characterize the texture of polycrystalline metals.
These measurements enable industry to replace destructive microscopic
inspections with nondestructive methods.

Of interest in velocity measurements are longitudinal
wave, which propagate in gases, liquids, and solids. In solids,
also of interest are transverse
(shear) waves. The longitudinal velocity is independent of
sample geometry when the dimensions at right angles to the beam
are large compared to the beam area and wavelength. The transverse
velocity is affected little by the physical dimensions of the
sample.

Pulse-Echo and Pulse-Echo-Overlap Methods

Rough ultrasonic velocity measurements are as simple as measuring
the time it takes for a pulse of ultrasound to travel from one
transducer to another (pitch-catch) or return to the same transducer
(pulse-echo). Another method is to compare the phase of the detected
sound wave with a reference signal: slight changes in the transducer
separation are seen as slight phase changes, from which the sound
velocity can be calculated. These methods are suitable for estimating
acoustic velocity to about 1 part in 100. Standard practice for
measuring velocity in materials is detailed in ASTM E494.

Precision Velocity Measurements (using
EMATs)

Electromagnetic-acoustic transducers (EMAT) generate ultrasound
in the material being investigated. When a wire or coil is placed
near to the surface of an electrically conducting object and is
driven by a current at the desired ultrasonic frequency, eddy
currents will be induced in a near surface region. If a static
magnetic field is also present, these currents will experience
Lorentz forces of the form

F = J x B

where F is a body force per unit volume, J is the
induced dynamic current density, and B is the static magnetic
induction.

The most important application of EMATs has been in nondestructive
evaluation (NDE) applications such as flaw detection or material
property characterization. Couplant free transduction allows operation
without contact at elevated temperatures and in remote locations.
The coil and magnet structure can also be designed to excite complex
wave patterns and polarizations that would be difficult to realize
with fluid coupled piezoelectric probes. In the inference of material
properties from precise velocity or attenuation measurements,
use of EMATs can eliminate errors associated with couplant variation,
particularly in contact measurements.

Fourier Transform-Phase-Slope determination of delta time between
received RF bursts (T2-R) - (T1-R),
where T2 and T1
EMATs are driven in series to eliminate differential phase shift
due to probe liftoff.

Slope of the phase is determined by linear regression of weighted
data points within the signal bandwidth and a weighted y-intercept.
The accuracy obtained with this method can exceed one part in
one hundred thousand (1:100,000).